Measuring devices of the generic type are known to be used to measure at least one process variable, in particular a mass flow rate, a density, a viscosity, a pressure or the like, in particular in a process line through which a medium can flow. They comprise at least one measurement sensor which provides a measurement signal that can be influenced by the process variable, the measured value determined for the process variable by the measuring device being able to be determined from a functional relationship in conjunction with measurement parameters from the measurement signal, and the measurement parameters having a temperature-dependent cross-sensitivity with respect to the temperature distribution in the measuring device. A Coriolis flowmeter which is known in principle is one specific example of a measuring device of the generic type.
The mass flow rate q and the mass density rho of a medium can be determined using a Coriolis flowmeter. In this case, (at least) one tube through which the medium flows is caused to oscillate in (at least) one Eigen mode at the Eigen frequency f and the temporal phase shift dt of the oscillation deflection is measured between two symmetrical points of the tube through which the flow passes. The mass flow rate q can then be calculated from the measurement signal of the phase shift dt usingq=K(dt−d0)and the density rho of the medium is obtained from the frequency f usingrho=rho0+W/(f2).
The abovementioned functional relationships for determining the measured values of the density rho and the mass flow rate q comprise the measurement parameters d0 (zero point phase), rho0 (density offset), K (flowmeter constant), W (density variation parameter).
The zero point phase d0, the density offset rho0 and also, in particular, the so-called flowmeter constant K and the density variation parameter W are now, however, dependent on the generally different temperatures of the individual components and spatial segments of the measuring device.
This means that, for example on account of temperature-induced stresses or changes in the material properties, each individual one of these parameters is a functional of the spatial temperature distribution in the measuring device d0=F1[T(x,y,z)], rho0=F2[T(x,y,z)], K=F3[T(x,y,z)] and W=F4[T(x,y,z)].
The problem is thus the cross-sensitivity of the flow measurements and density measurements with respect to the spatial temperature distribution T(x,y,z) and, after suitable discretization, with respect to an appropriate set of temperature values [T1, T2, . . . , Tn].
Although this cross-sensitivity can be calculated by measuring the temperatures T1, T2, . . . , Tn and with knowledge of the functional dependence of the parameters d0=f(T1, T2, . . . , Tn), rho0=f(T1, T2, . . . , Tn) etc., it requires a correspondingly extensive temperature measurement.
WO 04053428 discloses a method and a device which takes into account temperature values which have been recorded in the past, that is to say records a type of temperature history.
However, this includes only one partial aspect, namely that of monitoring whether a temperature shift may result in measured values being influenced locally.
In this case, the operations of recording and deriving temperature dependencies are therefore only inadequate, and may therefore have a corrupting influence.